Memory devices are typically provided as internal, semiconductor, integrated circuits in computers or other electronic devices. There are many different types of memory including random-access memory (RAM), read only memory (ROM), dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM), flash memory, and resistance variable memory, among others.
Memory can be volatile or non-volatile. Volatile memory requires power to maintain the information stored therein, e.g., when power to volatile memory is lost, the information stored therein is also lost. Non-volatile memory, in contrast, does not lose the information stored therein in the absence of power, e.g., non-volatile memory can retain the information stored therein even if no power is being provided to the memory. Types of volatile memory include RAM, DRAM, and SDRAM, among others. Types of non-volatile memory include ROM, flash memory, and resistance variable memory, among others.
Types of resistance variable memory include programmable conductor memory, phase change random access memory (PCRAM), and resistive random access memory (RRAM), among others. A physical layout of a PCRAM memory device can resemble that of a DRAM device, except that the capacitor of the DRAM cell is replaced by a phase change material, e.g., the memory cells of the PCRAM device consist of a phase change material, such as Germanium-Antimony-Telluride (GST). A physical layout of an RRAM memory device may include memory cells including a variable resistor thin film, e.g., a colossal magnetoresistive material, which can be connected to a current controlling device, such as a diode, a field effect transistor (FET), or a bipolar junction transistor (BJT).
The memory cell material of a PCRAM device, e.g., GST, can exist in an amorphous, high resistance state, or a crystalline, low resistance state. The resistance state of the PCRAM cell, e.g., the GST, can be altered by applying current pulses to the cell. For example, the resistance state of the PCRAM cell, e.g., the GST, can be altered by heating the cell with a programming current. This results in the PCRAM cell being programmed to a program level which corresponds with the resistance state of the cell. In a binary system, for example, the amorphous, high resistance state can correspond to a logic state of 1, and the crystalline, low resistance state can correspond to a logic state of 0. The resistance of an RRAM cell, e.g., the variable resistor thin film, can be increased and/or decreased by applying positive and/or negative electrical pulses across the film. This results in the RRAM cell being programmed to a program level which corresponds with the resistance of the cell.
A single level memory cell (SLC) can represent two programmed levels as represented by the binary digits 1 or 0. Memory cells can also store more than two binary digits, e.g., 1111, 0111, 0011, 1011, 1001, 0001, 0101, 1101, 1100, 0100, 0000, 1000, 1010, 0010, 0110, and 1110. Such cells may be referred to as multi state memory cells, multibit cells, or multilevel cells (MLCs). MLCs can allow the manufacture of higher density memories without increasing the number of memory cells since each cell can represent more than one digit, e.g., more than one bit. Further, MLCs can have more than one programmed state, e.g., a cell capable of representing four digits can have sixteen programmed states. Non-volatile resistance variable memory can achieve MLC devices by programming a memory cell to one of a range of resistances.
The resistance of a resistance variable memory cell can be sensed in order to determine the data state of the cell. During a sensing operation, the resistance of a selected memory cell can be compared to a reference resistance in order to determine if the memory cell has a greater or lesser resistance, thereby indicating the content of the stored data. One type of sensing operation of PCRAM cells utilizes a trimmable resistor stack located in the periphery of the PCRAM. In this sensing operation, the same magnitude of current is applied to both the trimmable resistor stack and the PCRAM cell. The voltages across the trimmable resistor stack and PCRAM cell are then compared to determine the state of the PCRAM cell, e.g., to distinguish a 1 state from a 0 state.
Prior to the aforementioned sensing operation, the trimmable resistor stack can be trimmed to provide a preset reference that emulates the GST resistance. However, the resistance characteristics of GST may vary significantly with temperature, and the trimmable resistor stack may not track the resistance of the GST as the GST undergoes a temperature change. Hence, if the GST undergoes a temperature change after the trimmable resistance stack is trimmed, the preset reference of the trimmable resistance stack may no longer emulate the resistance of the GST. This can cause errors in the sensing operation of the PCRAM, which can render the PCRAM unreliable.